Return-to-zero pulse sources are important components in wavelength division multiplexed optical fiber communications systems (WDM systems and dense WDM systems). Return-to-zero optical pulses (RZ pulses) are optical pulses whose power level drops to substantially zero. WDM systems, transmit optical signal pulses in a plurality of different wavelength channels. Dense WDM systems (DWDM systems) transmit more channels. RZ pulses, which typically have pulse widths on the order of 50 picoseconds or less, are the preferred optical pulses for WDM and DWDM systems, especially for long haul (long distance) transmission. Especially preferred are RZ pulses such as solitons that maintain their pulse shape integrity over long fiber lengths. As a consequence of their short duration and resistance to dispersion, solitons have been chosen as the preferred signal pulses for contemplated high speed (10 Gb/s and 40 Gb/s) long haul systems. Such systems will require inexpensive, compact, high power, jitter-free sources of soliton pulses.
While several RZ pulse sources exist, they all have technical drawbacks for these contemplated systems. Distributed feedback (DFB) lasers typically provide the optical power for generating soliton pulses. Gain-switched and filtered DFB lasers suffer from timing jitter, which limits transmission distance (Mollenauer, et al., Electronics Letters 27, 178-179(1991)). Mode-locked external cavity lasers require mechanical stability and have a repetition rate that is determined by the cavity length (Morton, et al., Institute of Electronics and Electrical Engineers (IEEE) Photonics Technology Letters, 5, 28-31 (1993)). Discrete electroabsorption (EA) modulators can carve out pulses from a CW signal, but they have a high 8-10 dB loss. An integrated laser/electroabsorption pulse source is potentially a viable solution. However, optical power, electrical bandwidth, and high contrast ratio remain challenges.
Present 10 Gb/s electroabsorbtive modulated lasers (EMLs), used for short-haul non return-to-zero (NRZ) transmission, are limited to −2 dBm to 0 dBm output power and have bandwidths of 11 GHz. Furthermore, output power has to be sacrificed for higher contrast ratio; the electroabsorption section has to be biased into the highly absorptive spectral region to achieve high contrast ratio.
At 10 GHz, the pulse source is typically a CW laser followed by a LiNbO3 modulator that is sinusoidally driven with high-power clock (>27 dBm). The main drawbacks of this combination are high loss of the modulator, large size, and high power consumption. The LiNb03 modulator has ˜5-6 dB of coupling loss in addition to the 3-5 dB loss suffered by the production of pulses from a CW signal. Because the modulator's large Vπ requires an ˜7 Vpp voltage swing it has a high power consumption. In addition, all of the above sources require a wavelength locker to achieve the wavelength stability demanded by DWDM systems. A discrete wavelength locker adds to the size, while integrated wavelength lockers add additional complication. Accordingly, there is a need for an improved soliton pulse source.